1. Field of the Invention
The present invention generally relates to substrate materials and processes suitable for use in electronic systems. More particularly, this invention relates to a multi-substrate circuit assembly capable of exhibiting enhanced electrical and thermal performance, particularly for purposes of improving thermal management of power circuit devices.
2. Description of the Related Art
A variety of approaches are known for dissipating heat generated by high-power integrated circuit (IC) devices. Power IC devices mounted to organic substrates such as a printed circuit board (PCB) can be in the form of a plated through-hole (PTH) package attached to a heat rail. A drawback of this approach is the use of PTH packages instead of the more desirable SMT (Surface Mount Technology) packages. This approach also requires the added assembly steps required to attach a heat rail, which are not easily automated and add significantly to product height. A thermal management method for SMT packages on organic substrates involves the use of thermal vias that conduct heat through the substrate to a heatsink. However, this approach has limited thermal capability since organic substrates are poor thermal conductors that can only be minimally enhanced with thermal vias. In contrast to PTH and SMT packages, whose package exteriors include thermal-insulating plastic that inhibits heat conduction directly through the package wall, enhanced thermal management of high-power IC flip chips mounted to PCB's has been achieved with heat-conductive pedestals in direct thermal contact with a surface of the device. Notable examples of this approach are disclosed in commonly-assigned U.S. Pat. Nos. 6,180,436 and 6,365,964 to Koors et al., which disclose pedestals that contact the non-active backside surface of a chip opposite the solder connections that attach the chip to the substrate. However, this and the preceding solutions preclude dual-sided heat sinking due to the organic layers of the substrate.
Ceramic materials such as beryllia (BeO), alumina (AlO3) and others have higher coefficients of thermal conductivity than organic substrates, and are therefore more often the substrate materials of choice for high-power IC chips. Because organic substrates offer excellent cost and conductor density benefits compared to ceramics, applications in which power IC devices are mounted to a ceramic substrate often include a PCB wire-bonded to the ceramic substrate to handle high-density routing requirements. However, this solution requires the more expensive serial process of wire-bonding. Furthermore, wire-bonds have assembly yield losses, current density issues, and adversely impact the critical electrical performance parameters of resistance and inductance. Finally, this approach is also limited by the inability to use dual-sided heat sinking because of the wire-bonds present on the surface of the power IC chip.
In view of the above, improvements in thermal management of power IC's would be desirable.